This project aims to investigate mechanisms of human selective attention by means of recording event-related brain potentials (ERPs) from the scalp in normal volunteer subjects. The ERPs are small voltage fluctuations in the ongoing EEG that are time locked to sensory and cognitive events and reflect the summated activity of neuronal populations engaged in the processing of information. Specific ERP components have been identified with processes of selective attention in both auditory and visual modalities. A series of seven experiments is proposed to increase our understanding of the physiological mechanisms that underly these attention-sensitive ERPs and the anatomical pathways that are involved. Experiments in both auditory and visual modalities will test for attention-related modulations in early evoked components that have been tentatively localized to subcortical or early cortical levels of processing. Inferences about the localization of neural generators will be supported by detailed topographical mappings of the voltage fields on the scalp. An overall goal in these experiments is to find out whether selective attention acts to gate or modulate sensory processing at an early (perhaps subcortical) level. Experiments with auditory attention will follow up our recent observations of a very early (20-50 msec) ERP change during dichotic distening to tone sequences. For visual-spatial attention, ERP recordings will be used to address the controversy of whether the improved processing of flashes at attended locations results from an early sensory facilitation or a subsequent change in decision criteria. The spatial extent of the "attentional spotlight" of improved processing will also be evaluated. This research relates to important mental health problems, since disturbances of selective attention are a characteristic of many clinical disorders including schizophrenia, attention deficit disorder and learning disability. The ERP studies proposed here should lead to an improved understanding of the basic mechanisms of both normal and disordered attention.